In underdoped cuprate superconductors, the Fermi surface undergoes a reconstruction that produces a small electron pocket, but whether there is another, as yet, undetected portion to the Fermi surface is unknown. Establishing the complete topology of the Fermi surface is key to identifying the mechanism responsible for its reconstruction. Here we report evidence for a second Fermi pocket in underdoped YBa2Cu3Oy, detected as a small quantum oscillation frequency in the thermoelectric response and in the c-axis resistance.

In the quest to increase the critical temperature Tc of cuprate superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field Hc2 is a fundamental measure of that strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. Here we show that the thermal conductivity can be used to directly detect Hc2 in the cuprates YBa2Cu3Oy, YBa2Cu4O8 and Tl2Ba2CuO6+δ, allowing us to map out Hc2 across the doping phase diagram.

Close to optimal doping, the copper oxide superconductors show 'strange metal' behaviour, suggestive of strong fluctuations associated with a quantum critical point. Such a critical point requires a line of classical phase transitions terminating at zero temperature near optimal doping inside the superconducting 'dome'.

Vortices in a type-II superconductor form a lattice structure that melts when the thermal displacement of the vortices is an appreciable fraction of the distance between vortices. In an anisotropic high-T c superconductor, such as YBa 2Cu 3O y, the magnetic field value where this melting occurs can be much lower than the mean-field critical field H c2. We examine this melting transition in YBa 2Cu 3O y with oxygen content y from 6.45 to 6.92, and we perform a quantitative analysis of this transition in the cuprates by fitting the data to a theory of vortex-lattice melting.

The electrical resistivity Ï c of the underdoped cuprate superconductor YBa 2Cu 3O y was measured perpendicular to the CuO 2 planes on ultrahigh quality single crystals in magnetic fields large enough to suppress superconductivity. The incoherent insulating-like behavior of Ï c at high temperature, characteristic of all underdoped cuprates, is found to cross over to a coherent regime of metallic behavior at low temperature.

The origin of pairing in a superconductor resides in the underlying normal state. In the cuprate high-temperature superconductor YBa2Cu 3Oy (YBCO), application of a magnetic field to suppress superconductivity reveals a ground state that appears to break the translational symmetry of the lattice, pointing to some density-wave order. Here we use a comparative study of thermoelectric transport in the cuprates YBCO and La 1.8-xEu0.2SrxCuO4(Eu-LSCO) to show that the two materials exhibit the same process of Fermi-surface reconstruction as a function of temperature and doping.

The Nernst coefficient of the cuprate superconductor YBa2Cu 3Oy was recently shown to become strongly anisotropic within the basal plane when cooled below the pseudogap temperature T, revealing that the pseudogap phase breaks the fourfold rotational symmetry of the CuO 2 planes. Here we report on the evolution of this Nernst anisotropy at low temperature, once superconductivity is suppressed by a magnetic field. We find that the anisotropy drops rapidly below 80 K, to vanish in the T=0 limit.

Over 20 years since the discovery of high temperature superconductivity in cuprates (Bednorz and Müller, 1986 [1]), the first convincing observation of quantum oscillations in underdoped YBa2Cu3O6.5 (Doiron-Leyraud et al., 2007 [2]) has deeply changed the theoretical landscape relevant to these materials. The Fermi surface is a basic concept of solid state physics, which underpins most physical properties (electrical, thermal, optical, etc.) of a metal. Even in the presence of interactions, this fundamental concept remains robust.